1. Field of the Invention
The present invention relates to ultrasonic motors.
This application is based on Japanese Patent Application No. 2006-212125, the content of which is incorporated herein by reference.
2. Description of Related Art
Recently, ultrasonic motors have been drawing attention as a new type of motor replacing electromagnetic motors. Ultrasonic motors have the following advantages over known electromagnetic motors:    1) Ultrasonic motors are capable of high torque without using gears;    2) Ultrasonic motors have holding force when powered off;    3) Ultrasonic motors have high resolution;    4) Ultrasonic motors are quiet; and    5) Ultrasonic motors do not generate magnetic noise and are unaffected by noise.
A known ultrasonic motor is described in Japanese Unexamined Patent Application, Publication No. HEI-7-193291. In this ultrasonic motor, internal electrodes for detecting vibrations are provided on some layered piezoelectric elements, which enables a vibration mode to be detected.
However, the vibration mode which can be detected using the ultrasonic motor described above is only a flexural vibration mode generated in a cylindrical rod-shaped vibrator. Accordingly, in a vibrator that simultaneously generates a longitudinal vibration mode and a flexural vibration mode, each vibration mode cannot be independently detected using the internal electrodes disclosed in Japanese Unexamined Patent Application, Publication No. HEI-7-193291; this is a problem because a separate, special device is thus required.